Change from the analog age to the digital age is rapidly progressing, as apparent from generalization of digital media, great and wide growth of electronic publishing industries, digitalization of diverse multimedia contents, and rapid development of digital communication networks such as the Internet, all of which have been recently made. That is, rapid and easy transfer and exchange of diverse data associated with e-books, Internet TV, images, videos, MP3, etc. are currently enabled. Using such multimedia services, therefore, users can rapidly and easily obtain desired information.
However, the change to the digital age involves various adverse effects. For example, the development of digital techniques has allowed a large number of copies to be produced. Furthermore, the development of communication networks has allowed the distribution of copies without any limitation. For this reason, creative works of individuals may be unreasonably used by stealth. Practically, such adverse effects have been highlighted as significant problems to providers, who provide data services such as MP3 files or moving picture data over the Internet, which must be surely eliminated.
Accordingly, research has been performed to provide various copy prevention techniques for effectively preventing copying of digital data, thereby protecting the copyrights of the digital data. For example, research has been actively performed in association with a digital watermarking method, which is known to be effective for prevention of copying of digital data. Watermarking is a technique developed to prevent copying of digital contents. In accordance with this technique, the owner of a copyright can embed, in a multimedia content created by him, a specific stream of digital data representing information about the ownership of the multimedia content while being visually and audibly imperceptible. Such a specific digital data stream is called a “watermark”.
This technique is very useful in that watermarks serve to easily determine whether or not digital data is under a copyright, while being visually and audibly imperceptible. For such a technique, various watermarking methods have been proposed which are mainly applied to DCT transform domains based on frequency domains.
For example, I. J. Cox has proposed a watermarking method in which the entire domain of an image is processed by DCT (Discrete Cosine Transform) without being divided into blocks so that random noise proportional to DCT coefficients are embedded, as watermark signals, in the domains, except for the low frequency domain. In addition, various watermarking methods based on DCT domains have been proposed. For example, a watermarking method based on block DCT has been proposed in which insertion of a watermark is determined based on a JND (Just Noticeable Difference) value using human visual characteristics. In accordance with this method, a product by the JND value is embedded as a watermark signal. Recently, a method has been proposed in which a visually-imperceptible watermark is embedded in a DC component of a DCT domain. Meanwhile, in pace with the recently increased demand for highly efficient compression of image and video data, research for compression of image data is actively conducted in association with image data compression using a wavelet transform, as compared to image data compression using a DCT, which involves a blocking phenomenon in the encoding of super-low-speed moving pictures. In particular, the watermarking methods based on DCT domains are ineffective in JPEG 2000, that is, a new image compression standard recently established for Internet environments, because compression of images is performed based on a wavelet transform in the JPEG 2000, different from the existing JPEG standards based on DCT.
Based on this background, research for watermarking methods based on wavelet transform, which are compatible with the JPEG 2000, is actively conducted.
Various watermarking methods based on the wavelet transform have been proposed. For example, there is a watermarking method in which watermark signals having different lengths are embedded in all high-frequency domains, except for the lowest-frequency domain, respectively. Also, a watermarking method has been proposed in which a watermark signal is embedded in a coefficient having a larger value. In most of the proposed methods, a watermark is embedded in frequency components, except for DC components, that is, the lowest-frequency components, after a frequency transform including a wavelet transform, taking into consideration the human visual characteristics more sensitive to a variation in low-frequency components than to a variation in high-frequency components. However, these methods still have problems in that the watermark is considerably damaged when the high-frequency components are eliminated in accordance with a compression process such as JPEG compression.
Furthermore, in fields where it is difficult to obtain original data such as media or moving pictures, for example, on the Internet, it is impossible to extract watermarks where there is no original data.
For this reason, it has been strongly required to provide a blind watermarking method based on wavelet transform which is capable of extracting watermarks without using original data, while enhancing a robustness of those watermarks to external attacks such as compression.
On the other hand, such a blind watermarking technique has been disclosed in an article “A Blind Watermarking Technique Using Wavelet Transform”, “ISIE 2001, Pusan, Korea”, pp 1946-1950, in 2000. This technique “A Blind Watermarking Technique Using Wavelet Transform”, only proposes a method in which characteristics of an image are used as watermarks. In other words, this technique provides no means for embedding and extracting random watermarks desired by users. Furthermore, this technique cannot achieve embedding of diverse watermarks because the number of pixels usable for a watermark in an image corresponds to only 1/10 of a typical watermark length. In particular, this technique can provide no means for extracting watermark data without using original image data.